ZEKE spectroscopy high Rydberg state

The other source of congestion is due to the molecular core. It is most readily discussed using the inverse Bom-Oppenheimer point of view to define the zero-order quantum numbers. Here each state of the ionic core has its very own series of high Rydberg states. The physical reality of this approximation is the observation [36,43] of the long-time stable ZEKE states not just below the lowest ionization threshold but also just below the threshold of ionization processes that leave an excited ionic core. Indeed, it is for this very reason that ZEKE spectroscopy is useful for the spectroscopy of ions (or for such neutrals that are produced by ionization of negative ions... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

As the potential for an ion-pair state is Coulombic, it will support an infinite number of vibrational states and these form a Rydberg series close to the dissociation threshold. These weakly bound states appear to behave like the high- Rydberg states employed in ZEKE photoelectron spectroscopy, described in the previous section, and they have, therefore, been termed zero ion kinetic energy (ZIKE) states (Wang... 

Schlag, E. W. and R. D. Levine (1997). ZEKE spectroscopy of ions, radicals, reactive intermediates, and clusters and the dynamics of high molecular Rydberg states. Comm. At Mol. Phys. 33, 159. 

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